Three-year statute of limitations from the date you file the original return for the IRS to assess taxes against you - or 2 years from the date you paid your taxes.
Notwithstanding, the time period changes:
6 years if you failed to report greater than 25% of the income shown on your return - a substantial under-reporting of your income.
no time limit for auditing and collecting taxes if you file false/fraudulent returns.
10 years from the date of assessment for the IRS to collect.